1. Field of the Invention
The present invention relates to an optical disc recording method and apparatus for recording, by optical power, information on an optical disc, such as CD-R, CD-RW, CD-WO, MD and DVD. More particularly, the present invention relates to detection of a wobble signal at the time of recording.
The present application is based on Japanese Patent Application No. Hei. 11-285820, which is incorporated herein by reference.
2. Description of the Related Art
The write-once optical disc, e.g., CD-R (i.e., Compact Disc-Recordable) disc and a rewritable optical disc, e.g., CD-RW (i.e., Compact Disc-ReWritable) disc have guide grooves, so called as grooves or pre-grooves, which are formed in advance in a recording layer thereof. By irradiating a laser beam onto and along a groove or a track formed with a land which is defined between the adjacent grooves formed in an optical disc which has been rotated, information is recorded on the track in a form of strings of pits. Absolute time in pre-groove information (i.e., ATIP information) is buried in the groove of the optical disc of this type by wobbling the track groove. Ordinarily, the recording/reproducing operation is controlled, based on a tracking servo and the ATIP information, which is obtained by reproducing the wobble signal.
As shown in FIG. 5, a tracking error signal (TE signal) and a wobble signal are detected by processing an output signal of a position-detecting photo detector of an optical pickup, which is ordinarily a four-segment photo detector 101. The four-segment photo detector 101 has an imaginary plane having four light-receiving segmental areas A, B, C and D. According to those light receiving areas, the areas (A+D) and (B+C) form a light receiving surface divided in the radial direction of the optical disc, and the areas (A+B) and (C+D) form a light receiving surface divided in the track direction of the optical disc. The light receiving areas A, B, C and D produce signals when those areas receive optical beams reflected from the optical disc. Those output signals are respectively sampled and held by sample/hold circuits 102a, 102b, 102c and 102d. The signals, which have been derived from the light receiving areas A and D, and sampled and held by the sample/hold circuits 102a and 102d, are added together by an adder 103. The signals, which have been derived from the light receiving areas B and C, and sampled and held by the sample/hold circuits 102b and 102c, are added together by an adder 104. A difference between the output signals of the adders 103 and 104, i.e., ((A+D)xe2x88x92(B+C)), is computed by a subtractor 105. EFM signals are modulated by presence or absence of pit detected in phase at the light receiving surfaces A, B, C and D. The signals derived from the light receiving areas (A+D) and (B+C) have inverse phases with respect to a position as viewed in the radial direction of the beam spot on the land. Accordingly, those signals can be utilized as TE signals or the wobble detecting signals.
FIG. 6 is a graph showing a relationship among a recording signal, a light receiving signal, pits formed, and a wobble-signal detecting sampling pulse signal when information is recorded at a speed of four times relative to the standard audio CD speed (hereinafter, referred simply as a standard speed). As shown in FIG. 6, a waveform of the recording signal has a xe2x80x9crecording partxe2x80x9d when its power is in an on state, and a xe2x80x9clead partxe2x80x9d when its power is in an off state. A level of the light receiving signal varies in accordance with a reflectivity variation during a recording pit forming process, to thereby have a waveform pattern as follows. The signal level of the light receiving signal reaches its peak immediately after the recording signal is put in a power-on state. Then, the signal level gradually descends with formation of a pit. When the recording signal is put in a power-off state, its level variation is settled down at a fixed level for a part of the period, to thereby form a flat waveform pattern. That is, the pit formation begins at an instant that the light receiving signal level reaches the peak value, and ends some time after the recording signal is put in a power-off state. A wobble sampling pulse signal is held for a fixed period of time after the recording signal is put in a power-off state. The reason why it is held for the fixed time is that if the wobble detection sampling is performed during a period where the light receiving signal is stable in level variation, the carrier to noise (C/N) ratio of the wobble signal is good.
In the wobble detecting method and apparatus described above, when information is recorded at a speed, which is higher than the above-described speed, and which is, for example, twelve times relative to the standard speed, the waveform pattern of the light receiving signal greatly varies, and the stable period during the power-off period of the recording signal is extremely short. The first reason is that in the case of the high speed recording, in order to secure a reliable pit formation, a recording time must be increased with increase of the recording speed. For example, provided that the recording time is nT when the recording speed is eight times relative to the standard speed, a recording time at the speed of twelve times is (n+0.4)T. Further, the second reason is that in the high speed recording, the recording power is larger than that in the lower speed recording, and therefore, the light receiving signal is more dull in waveshape. Accordingly, in the case of the high speed recording, the sampling time period for the wobble detection is extremely short, so that the C/N of the wobble signal is degraded.
It is therefore an object of the present invention to provide an optical disc recording method and apparatus which can detect a wobble signal at all times at good C/N ratio independently of the speed recording.
According to the first aspect of the present invention, there is provided an optical disc recording method including the steps of:
irradiating an optical beam for information recording onto an optical disc, based on a recording signal;
forming at least one pit having predetermined length on a land disposed on the optical disc;
obtaining a light receiving signal by causing a light receiving element, having a light receiving surface segmented in a radial direction of the optical disc, to receive light reflected from a pit forming part and a non-pit forming part, both of which are formed on the optical disc at the time of information recording;
sampling and holding the light receiving signal derived from the light receiving element; and
detecting a tracking servo and a wobble signal on the basis of balance of the sampled and held light receiving signal as viewed in the radial direction of the optical disc;
wherein when a recording speed is lower than a predetermined recording speed, the sampling and holding step is performed to detect the wobble signal during a power-off period of the recording signal, and
wherein when the recording speed is higher than the predetermined recording speed, the sampling and holding step is performed during a power-on period of the recording signal.
According to the second aspect of the present invention, there is provided an optical disc recording apparatus which includes:
an optical pickup emitting an optical beam to record a pit onto a land of an optical disc, based on a recording signal, the optical pickup including a light receiving element which has a light receiving surface segmented in a radial direction of the optical disc, and which receives light reflected from the optical disc, the optical pickup producing a light receiving signal associated with each segmental area of the light receiving surface;
a sampling pulse generating circuit processing the recording signal to generate a first sampling pulse signal, the sampling pulse generating circuit retarding the recording signal to generate a second sampling pulse signal which is put in an on-state of the recording signal when the recording speed is lower than a predetermined speed, and which is put in an off-state of the recording signal when the recording speed is higher than the predetermined speed;
a first sample/hold circuit which samples and holds the light receiving signal derived from the optical pickup based on the first sampling pulse;
a tracking error detecting circuit which generates a tracking error signal based on balance of the light receiving signal sampled and held by the first sample/hold circuit as viewed in the radial direction of the optical disc;
a servo circuit which performs a tracking control of the optical pickup on the basis of the tracking error signal from the tracking error detecting circuit;
a second sample/hold circuit which samples and holds the light receiving signal output from the optical pickup based on the second sampling pulse; and
a wobble detecting circuit which detects a wobble signal based on balance of the light receiving signal sampled and held by the second sample/hold circuit as viewed in the radial direction of the optical disc.
When an optical beam is irradiated onto an optical disc having a dye layer (i.e., when a recording signal is in a power-on state), a reflecting light signal (i.e., light receiving signal) from the optical disc is ordinarily high in level immediately after the irradiation since a pit is not formed on the optical disc and hence its reflectivity is high. As the pit formation progresses at a time constant, the reflectivity gradually decreases. Accordingly, the light receiving signal likewise decreases in level. When the optical beam is not irradiated (i.e., when the recording signal is in a power-off state), a time period appears where the light receiving signal is stable, viz., a curve representing a variation of its signal level is substantially flat. This phenomenon occurs when the recording speed is lower than a predetermined recording speed. For this reason, in the present invention, when information is recorded at a recording speed, which is lower than a predetermined recording speed, the sampling operation for detecting a wobble signal is performed during a period where the light receiving signal is stable in level variation within a power-off period of the recording signal. With increase of the recording speed to a high speed of, for example, ten times as high as the standard speed, it is required to increase a ratio of the power-on period to the power-off period of the recording signal in order to reliably form pits. Additionally, the light receiving signal is dull in waveshape at the leading and trailing regions (i.e., its output level descending is gentle and its bottom peak appears with a delay). As a result, the period where the light receiving signal is stable in level variation within the power-off period of the recording signal is short, and hence the sampling period is reduced. For this reason, in the present invention, when the recording speed is higher than the predetermined one, the sampling operation for wobble detection is performed during the recording period. By doing so, the level of the light receiving signal per se is large, and as a whole, a wobble signal can be detected at good C/N ratio independently of the recording speed.
In the optical disc recording method, the timing of the sampling/holding operation of the light receiving signal for detecting the wobble signal may adaptively be changed at the predetermined recording speed in accordance with a kind and a recording speed of the optical disc. If the timing of the sampling/holding operation is so changed, an optimum wobble signal C/N can be obtained at all times even if a kind and a recording speed of the optical disc are changed to other ones. As a result, the wobble servo is more stabilized.
The sampling pulse generating circuit of the optical disc recording apparatus according to the present invention preferably includes:
a first delay element which retards the first sampling pulse signal;
an inverter which inverts the first sampling pulse signal;
a second delay element which retards the first sampling pulse signal inverted by the inverter;
a switch which selects one of output signals of the first and second delay elements, and which outputs the second sampling pulse; and
a switching control circuit which adaptively controls the switching operation of the switch based on at least the recording speed.